Revised:

The smallest of the URF’s, URFA6L, has been identified as the animal equivalent of the recently discovered yeast W-ATPase subunit 8 gene; but the functional significance of other URF’s has been more elusive. Recently, however, several human URF’s have been shown to encode subunits of rotenone-sensitive NADH-ubiquinone oxido-reductase. This is a large complex that also contains many subunits synthesized in the cytoplasm; it will be referred to hereafter as respiratory chain NADH dehydrogenase or complex I. Six subunits of Complex I were shown by enzyme fractionation studies and immunoprecipitation experiments to be encoded by six human URF’s (URF1, URF2, URF3, URF4, URF4L, and URF5); these URF’s will be referred to subsequently as ND1, ND2, ND3, ND4, ND4L, and ND5.